Conventionally, a plate thickness correction tool for reduction of a molding defect at a corner portion having an L-shaped or C-shaped cross section has been known (see, for example, Patent Literature 1). This plate thickness correction tool is placed over composite materials, which have been shaped so as to have an L-shaped or C-shaped cross section, and the plate thickness correction tool is made of a material having flexibility at a temperature lower than a heat curing temperature of the composite materials. The plate thickness correction tool uniformly maintains plate thickness of the composite materials with elastic force of elastic deformation in a state that the plate thickness correction tool is placed over the composite materials even after the composite materials cured.